1. Field of the Invention
This invention relates to an internal power supply circuit of a semiconductor integrated circuit, and in particular relates to an internal power supply circuit having a cascode current mirror circuit, and to a startup circuit for cascode bias.
2. Description of the Related Art
In order to attain both fast operation and low power consumption, semiconductor integrated circuits such as DRAMs have internal power supply circuits which generate an internal step-down power supply from an external power supply. Internal circuitry operates by means of the internal step-down power supply. In the case of DRAM, the internal power supply circuit generates an internal step-down power supply VII lower than the external power supply VDD. The internal power supply circuit also generates an internal step-up power supply VPP higher than the external power supply VDD.
The internal step-down power supply VII is supplied to the DRAM peripheral circuits, and is also supplied to the memory core having a memory cell array. Also, the internal step-up power supply VPP and a separate internal step-down power supply lower than the internal step-down power supply VII are supplied to the memory core. The internal step-down power supply VII and internal step-up power supply VPP are generated from the external power supply VDD.
For the above and other reasons, during power supply startup the internal power supply circuit monitors the rise of the external power supply VDD, and after detecting the rise, starts up the internal step-down power supply generation circuit and internal step-up power supply generation circuit. In addition, the rise of the internal step-down power supply VII is monitored, and after detecting the rise, the separate internal step-down power supply generation circuit is started. Upon detecting the rise of all internal power supplies, the internal power supply circuit outputs a start signal indicating that the series of internal power supply startup operations has ended. In response to this start signal, the internal circuit begins operation. In order to control the power supply startup operation sequence, the internal power supply circuit has a sequencer.
Internal step-down power supply generation circuits are disclosed in the following patent references. In Japanese Patent Laid-open No. 2001-28188, a circuit which generates two internal step-down power supplies is disclosed; in Japanese Patent Laid-open No. 09-62380, an internal step-down power supply circuit is disclosed.
The internal power supply circuit has for example a current mirror circuit in a differential amplifier or similar to detect the internal power supply potential. A current mirror circuit generates a current proportional to the size of each transistor by applying a common potential to the gate of a transistor provided in parallel on the power supply side. By adopting a current mirror circuit as the load circuit for the differential amplifier, equal currents can be supplied to a pair of differential transistors, so that offsets and mismatches are prevented.
Transistors which have become increasingly smaller in size in recent years have characteristics in which, in the saturation region, the drain current depends on the drain-source voltage. A cascode structure has been proposed for current mirror circuits to accommodate this fact. By employing a cascode structure, the drain potentials of the current source transistors provided in parallel with the current mirror circuit can be made equal, and the drain-source voltages can be kept equal, so that current mirror circuits can be made to operate normally.
However, there is the problem that, when an external power supply rises when turning on power, the cascode bias potential supplied to a cascode-connected transistor is temporarily higher or lower than the ordinary potential. In the normal operating state, it is required that a constant potential difference be maintained between the cascode bias voltage and the power supply serving as reference; but at power-on, the potential changes to a potential different from the potential in the normal operating state, impeding normal operation of the cascode current mirror circuit. As a result, anomalies occur in the circuit operation of differential amplifiers and other circuits having a cascode current mirror circuit. For example, anomalies occur in circuits generating an internal step-down power supply VII and in circuits generating an internal step-up power supply VPP, so that there is overshooting of the internal step-down power supply VII or internal step-up power supply VPP, greatly detracting from the reliability of devices connected thereto.